The present invention relates to a gas-insulated electrical apparatus, and especially to a gas-insulated electrical apparatus suitable for an electric power substation or a switching station, which is capable of reducing DC residual voltage and preventing an insulation breakdown.
A gas-insulated electrical apparatus is composed of bus-bars contained in tanks into which insulation gas is sealed, bus-bar disconnectors, circuit breakers, and line circuit breakers, and is connected to transmission-lines or transformers via bushings.
Each bus-bar circuit breaker and each line circuit breaker perform a charging-current interruption which interrupts power fed to a capacitive load by voltage-interruption between a fixed-contact and a moving-contact which are included each circuit breaker, which is carried out by parting the moving-contact from the fixed-contact, and perform a loop-current interruption in which a current-flowing route is switched by communicating the current-flowing route by means of arch-discharging. However, while the moving-contact is parted from the fixed-contact, the re-ignition of arc occurs, which causes a surge of voltage.
As an apparatus which is capable of reducing a surge voltage while parting a moving-contact from a fixed-contact by preventing the re-ignition phenomenon occurring during the charging-current interruption, Japanese Patent Application Laid-Open Hei 2-44620 discloses an apparatus in which an auxiliary moving-contact is provided in a main moving-contact, a pressing member which moves in accordance with motion of the main moving-contact presses a pressed member located in a drive member for driving the auxiliary moving-contact, and the pressed member further presses the drive member.
In the conventional gas-insulated electrical apparatus, when an opening operation is performed, at first, a moving-contact in a circuit breaker is parted from a fixed-contact to interrupt current. However, since the contact-parting speed is a high speed of the order of several m/s, the contact-parting operation (opening operation) is completed without the re-ignition between the fixed and moving-contacts. Next, a disconnector is opened, and the re-ignition of arc occurs during the opening operation of the disconnector, which causes surge-voltage. Further, DC voltage remains at high-voltage conductors in the circuit breaker and the disconnector. The level of this residual voltage depends on the structure of the disconnector and the contact-parting speed, and if the level of the residual voltage is high, the insulation between the contacts of the disconnector may break down. In the worst case, a reverse-polarity DC residual voltage remains between the contacts, and its value is about 2 pu, where 1 pu is a peak value in the ordinary AC voltage to the ground level, and it is required that this residual voltage be decreased.
The residual voltage remaining between the circuit breaker and each disconnector due to the opening operation of the disconnector depends on the structure of the disconnector and the contact-parting speed in the disconnector. Since it is usually necessary to interrupt a bus-bar loop current, an electromotive spring operation method is adopted in the contact-parting operation of the disconnector, and the contact-parting speed is set at about 1-3 m/s. At the speed in this range, the re-ignition of arc is repeated between the tops of the fixed-contact and the moving-contact, and the insulation recovery voltage increases while the distance between the contacts increase. Thus, a large residual voltage of maximally 1 pu to the ground level and of maximally 2 pu between the contacts in the disconnector remains as a residual DC voltage after the opening operation of the disconnector has terminated.
Generally, in the insulation recovery voltage, the value of the insulation recovery voltage increases with respect to the distance between the contacts, and a positive voltage value is larger than a negative voltage value. Accordingly, a positive insulation recovery voltage remains after the opening operation has terminated. However, at a high contact-parting speed of about 1-3 m/s, the residual voltage becomes either positive or negative depending on the phase at the start of the contact-parting operation. Therefore, the residual voltage in the disconnector is large, and its polarity becomes one of the positive or negative polarity in the same probability. Thus, the insulation between the contacts, or between the conductors and the tank, probably breaks down.
If a conductor between the circuit breaker and each disconnector possesses a positive potential, a metallic extraneous substance in the tank reciprocates between the conductor and the inside surface of the tank, and it is highly probable that the charged extraneous substance adheres to the coated inside surface of the tank.
However, if a negative voltage remains between the circuit breaker and each disconnector, and the conductor possesses a negative potential, a floatable extraneous substance floats up and reaches the central conductor. Further, the extraneous substance also possesses a negative potential, and moves down due to the electrostatic repulsive-force and the gravity force. Furthermore, electrons are emitted from the extraneous substance due to local discharge during moving-down of the extraneous substance, and the polarity of the extraneous substance becomes positive. Finally, the extraneous substance is attracted by and adhered to the conductor. If there is an extraneous substance near a conductor with a high voltage, and a high voltage due to a thunder surge, etc., is applied to the extraneous substance and the conductor, discharging is induced by the extraneous substance, and a grounding accident possibly occurs.